Thin printing papers are used mainly for printing reference works and catalog-type works, for example, telephone directories, mail order catalogues, and the like. In principle, a distinction is made between two types of paper, namely, the so-called natural, or uncoated, papers that have no special surface coating, and coated papers that today generally have a coating of pigment that is bonded by a synthetic bonding agent. The latter types of paper are referred to as LWC papers (low-weight coated) or, when they are of the lowest weight range per unit area, as ULWC papers (ultra-low-weight-coated). These are the type designations of standard qualities that are commercially available. When they are used, in particular, for rotogravure printing, both natural papers as well as the coated printing papers are super-calendered in order to impart the specific smooth finish that is required for the printing process. The standard designation for super-calendered papers is SC-papers. As a rule, these papers are suitable both for rotogravure printing and for off-set printing. This is not the case with known coated papers. In the case of conventional LWC-qualities, additional information must be provided as to whether they are to be used for rotogravure printing or for offset printing. Coated papers that are used for rotogravure printing must be sufficiently pliable in order that they lie properly on the inking depressions of the rotogravure printing cylinder; because of the easy flowing characteristics of the offset inks, offset papers require great surface strength. These different demands require different manufacturing conditions and, generally speaking, cannot be achieved simultaneously using known manufacturing processes.
Papers with ever-smaller weights per unit area are demanded not only for environmental reasons, in order to reduce the amount of waste paper that is generated, but are demanded mainly in order to save freight costs when transporting paper and to reduce mailing costs when sending out printed matter, for example, mail-order catalogues, for in the case of light-weight paper, the information area is then greater for each unit of weight of the paper.
There are two reasons why there are limits to which the weight per unit area of printing papers can be reduced: on the one hand, for reasons of a still-acceptable strength of the paper, which is important both when the paper is manufactured in a paper-making machine as well as when it is being printed using other high-speed rotary presses; on the other hand, for reasons of the required printing opaqueness, which must not fall below a specific value in order that the paper can be printed on both sides without the image bleeding through to the other side. Generally speaking, a high level of opaqueness can be achieved by having a greater quantity of mechanical wood pulp or pigment in the body paper, or a greater weight of coating on the paper, although this affects the strength of the paper in the case of equal weights per unit area because the proportion of long-fibre paper fibres material (which imparts strength to the paper) must, as a rule, be reduced in such a case.
Coated papers cost more to manufacture than uncoated papers but they have a smoother surface that results in a better print image; however, the synthetic binder, in the form of a hardened polymer dispersion used in the coating substance, is a disadvantage for environmental reasons, and also for re-processing. Very frequently, coated papers that are produced at very low weights per unit area are spongy because of the small percentage of base paper that they contain and also because of the synthetic binding agent that is used, and this can be a disadvantage for handling the paper. If, instead, use is made of an uncoated paper at a low weight per unit area, printing opaqueness and printing brilliance (printing brightness) are reduced and more printing ink bleeds through. Since these known papers have no subsequently applied coating, their opaqueness can only be improved by increasing the amount of filler or mechanical wood pulp in the paper itself. As has been discussed above, this reduces the strength of the paper. However, the strength of the paper is determined not only by the type and processing of the fibre material in the paper, but also by the evenness of the sheet forming in the paper-making machine, because the better the evenness of the paper, the smaller the number of weak points, which are not as strong, there will be; in the final analysis, these cause breaks or tears in the paper web. Generally speaking, uneven sheet forming results in uneven absorption of the printing ink, so that the integrity of the printed image suffers.
For reasons of cost, bulk printing papers are not manufactured from pure pulp, which would give the greatest paper strength, but rather the largest possible quantity of mechanically or thermo-mechanically digested mechanical wood pulp or wood cellulose is used, since this not only entails cost advantages but also improves the opaqueness of the paper and has a positive effect on the printing results that can be achieved. Mechanical wood pulp reduces the strength of the paper that can be achieved or which could be achieved by using pure pulp. In addition, the price situation with respect to bulk printing papers is such that such papers can only be produced economically on very powerful and high speed production machines. Depending on the particulars of the production plant that is used, the raw-material costs relative to unit area for lower paper weights can be reduced, whereas the cost of each unit of weight of the paper that is produced can increase since the reduction of weight per unit area cannot be made up in every case by increasing the speed at which the machines run, in order to maintain even production with respect to weight.